This invention relates to union fittings. More particularly, this invention relates to lip seal union fittings for control valves, hydraulic cylinders, reservoirs, manifolds, and other hydraulic applications.
Hydraulic components such as pumps, reservoirs, control valves, manifolds, hydraulic cylinders, and other hydraulic applications employ straight thread port connections and tapered pipe-thread port connections. Straight thread port connections are typically called boss ports or bosses.
Straight-thread bosses include a chamfered face surface or seat and a threaded bore. The union fitting includes a body with a threaded tube-side end, a threaded boss-side end, and a central portion with a hexagonal outer surface for engaging a wrench. To connect a tube to the boss, the boss-side end of the union fitting is inserted into the threaded bore of the boss. A cavity is formed at an interface between the seat of the boss and the boss-side end of the union fitting. An elastomeric O-ring is installed over the boss-side end and is moved against the central portion. As the union fitting is tightened, the elastomeric O-ring distorts and fills the cavity to create a fluid seal.
The elastomeric O-ring is distorted as the union fitting is tightened onto the boss. The strain on the elastomeric O-ring causes accelerated aging which, in turn, causes early O-ring failures. During use, the cyclical application and release of fluid pressure further contributes to the deterioration of the O-ring. In high pressure or high temperature applications, the elastomeric O-ring undergoes further wear and tends to lose the fluid-tight seal more quickly. Despite the problems associated with union fittings which employ elastomeric O-rings, the aerospace industry still uses them pervasively.
Various different union fittings have been proposed to eliminate the problems associated with elastomeric O-rings. A lip seal union fitting includes a metal lip that is integrally formed with the body of the fitting. The metal lip extends from the central portion and contacts the chamfered face surface or seat of the boss as the fitting is tightened. The integral metal lip has sufficient flexibility to overcome the lack of roundness or concentricity of either the boss or the union fitting. The integral lip seal provides a continuous metal-to-metal seal when the lip seal union fitting is tightened onto the boss. The lip seal union fitting is typically made of aluminum alloy, steel, or titanium alloy.
In some applications, however, steel hydraulic tubes are employed. The lip seal union fitting cannot be made from aluminum alloy because of galvanic corrosion coupling that occurs when dissimilar metals are in contact and because of strength and fatigue resistance issues. If the union fitting is made of steel, stainless steel or titanium alloy, the flexibility of the integral metal lip is reduced and the weight of the union fitting is increased. Successful applications of lip seal union fittings generally require that the boss be constructed of a material which is at least as hard as the material used in constructing the union fitting. Most of the bosses into which the union fittings are assembled are made of aluminum or aluminum alloy. Aluminum alloy, steel and stainless steel may also be used for the boss. Therefore, the metal used for the union fitting should not be harder than aluminum or aluminum alloy. Otherwise, the union fitting (which is a less expensive part) may damage the boss (which is the more expensive part) if the connection between the union fitting and the boss is over-tightened.
Some conventional union fittings with integral metal lips were made from titanium alloy which is the next most flexible metal after aluminum. When titanium alloy is employed, slight flexing of the integral metal lip occurs with each pressure application. Eventually, the flexing causes galling at a contact ring between the boss and the union fitting. The galling allows fluid leakage. Corrective actions involve reworking a cavity between the metal lip and a shank of the union fitting and installing an elastomeric O-ring as a redundant seal.
A lip seal union fitting according to the invention connects to a female boss that is associated with a hydraulic component. The female boss includes a bore, a first threaded portion located on an inner surface of the bore, and a seat. The lip seal union fitting includes a body that is constructed of a first material. The body includes a tube-side end, a boss-side end, and a central portion. A second threaded portion is located on an outer surface of the boss-side end. A lip seal is constructed of a second material. The lip seal is attached to and extends from the central portion of the body towards the boss-side end. The lip seal includes an outer surface that engages the seat of the boss to create a fluid seal as the first and second threaded portions are rotatably engaged. The first material has a greater material hardness than the second material.
In one feature of the invention, the lip seal is attached to the central portion using inertia welding.
In another feature of the invention, the boss is made of a third material. The second material has a hardness that is approximately equal to the third material.
In another feature of the invention, the first material includes at least one of steel, stainless steel, and titanium alloy. The second material includes at least one of aluminum and aluminum alloy.
In still another feature of the invention, the lip seal is cantilevered from the central portion. The central portion includes a first flange that projects in a radial direction from the body and a second flange that projects in an axial direction from a boss-side surface of the first flange. The lip seal is connected to the body along the second flange.
A method of making a lip seal union fitting according to the invention includes the step of constructing a body of the first material that has a bore-side end, a tube-side end, and a central portion. An outer surface of the boss-side end is threaded. A lip seal is constructed of a second material. A hardness of the first material is selected such that it is greater than a hardness of a second material. The lip seal is attached to the central portion of the body.
In another feature of the invention, the step of attaching includes the step of welding a part to the central portion. The part has an inner surface with a shape corresponding to a shape of an inner surface of the lip seal. An outer surface of the part is machined to form the lip seal. A finish is applied to the lip seal.
In still another feature of the invention, the central portion is provided with a first flange that projects in a radially outward direction from the body. An annular projection is formed on a boss-side surface of the first flange. The lip seal is attached to the annular projection.
Other objects, features, and advantages will be readily apparent.